This invention relates generally to the agglomeration, pelletizing, or granulation of various plant food nutrients, either as complete fertilizer products suitable for direct field application, or as granular concentrates which can be physically blended with other materials to form complete fertilizer formulations. More specifically, this invention relates to use of a novel binder/conditioner to assist in the agglomeration, granulation or pelletizing of various fertilizer components.
Most fertilizer granulation plants still operating in the United States are of the continuous ammoniation-granulation type. In such a plant superphosphate, potassium chloride, potassium sulfate, ammonium sulfate, urea and other dry nutrient containing materials are measured continuously by gravimetric belt feeders. Liquid raw materials such as anhydrous ammonia, nitrogen solutions, sulfuric acid and phosphoric acid are measured continuously by metering. Both liquid and dry materials are fed into a rotating drum or granulator, where products are intimately mixed and agglomeration takes place. After the initial granulation phase, the product passes through a rotary dryer and rotary cooler. The cooled product is screened to separate the on-size product from oversize and fines. Oversize material is crushed and rescreened, and fines are recycled back to the ammoniator-granulator. The Tennessee Valley Authority (TVA) and the United States Department of Agriculture have contributed a wealth of additional information to the granulation process over the past three decades. Circular Z-18, published in November 1970 by the TVA and The National Fertilizer Development Center, entitled "Producing Granular Fertilizers," describes in detail typical granulation techniques of fertilizers.
Raw materials used in the granulation process usually vary in size from as small as, for example, minus 325 mesh to as large as the minimum size specification of the finished product. Great care must be exercised in determining the proportions of various sized raw materials to be included in granulating product, so as to produce a final mix which granulates at an efficient rate. Maximum level of on-size finished product, particularly when no binder is used, is produced when a variety of sizes of raw material is included in the raw material feed. This, however, tends to significantly increase the cost of final product. At the same time, nitrogen, phosphate, potassium and secondary and micronutrient levels must be maintained at the required specification in order to meet the desired formulation.
Many raw material products available to the fertilizer granulator are more economically priced if the granulator can use reduced size or "fines" of the raw material in question. Raw materials such as potassium chloride, potassium nitrate, ammonium sulfate, ammonium phosphate, urea, potassium sulfate, and others, are frequently available to the fertilizer granulator in a reduced sizing and at a reduced cost. Maximum economy in such cases depends on maximum use levels. In addition, there is often raw material available which is considered "distressed goods," as it is no longer in condition to be used for bulk blended product.
If a proper balance of fertilizer chemistry and raw material size balance is not met, the granules which are formed during granulation are often weak and readily disintegrate to powder during handling and use. Dustiness in the product can become excessive, causing extreme handling and flow problems during transfer and in fertilizer application equipment.
Certain other raw materials and raw material combinations, when used at the most economically desirable levels, draw excessive moisture to the finished granule, thus limiting its storage and shelf life and causing excessive caking in storage bins, finished product piles and fertilizer application equipment. One class of such ingredient which frequently exhibits this characteristic is urea and urea combinations. A granular urea containing composition encompassing calcium sulfate and calcium phosphate is shown in U.S. Pat. No. 4,283,423, of Watkins et al. Various possibilities have been proposed to avoid such caking when utilizing urea in fertilizer manufacture. They include "dusting" a conditioning agent on the manufactured granule. Such dusting agents include starch, clays and calcium sources, as described in U.S. Pat. No. 3,332,827. This method, however, is less than satisfactory as it requires additional equipment and at best only coats portions of the granule thus providing only partial relief from caking.